The present invention relates to a golf wood club head and a method for producing the same, and more particularly relates to improvement in construction and production of a golf wood club head having a hollow metallic shell construction.
In general, it is very difficult for beginner level golfers to maintain stable swing at striking balls and unstable swing causes a ball to come into impact contact with various sections around the sweet spot on a club head. Incorrect impact contact of the ball with the sweet spot often results in error in shot and, even when shot itself is performed without fail, ball tends to fly in an unintended direction. The more one intends to have correct shot on the sweet spot, the less one can swing the club strongly and such suppressed swing only ends in short flying distance.
In order to make up for such poor manipulation at striking balls, it is broadly wanted by beginner level golfers to have a club head with an enlarged sweet spot and a light weight construction.
An enlarged sweet spot assures constant positioning of its impact contact with balls even with unstable swing by beginner level golfers and, as a consequence, the ball flies in an intended direction. In addition, the enlarged sweet spot mentally allows a golfer to swing the club stronger in order to have a long flying distance of the ball.
Golf wood club heads now in market and practical use are roughly classified into three types, i.e. a metallic head having a shell made of cast metal such as stainless steel and titanium, a wooden head having a shell made of woods such as persimmon and a CFRP club having a shell made of plastics reinforced by fibers such as carbon fibers.
In order to enlarge the sweet spot on such a wood club head, it is theoretically thinkable to increase the entire size, i.e. the volume, of the club head, thereby raising its moment of inertia. In practice, however, an increased size is inevitably accompanied with an increased weight of the club head which allows no quick swing at striking balls in particular in the ease of a golfer of a low physical abilities. Slow swing does not provide a strong impact on a ball at striking and, as a consequence, the ball cannot fly over a long distance.
The total weight of a club head has an upper limit of about 210 g in the case of general golfers of an ordinary physical abilities and presence of such an upper limit in weight bars limitless enlargement of the sweet spot on a club head. Enlargement in sweet spot is suppressed from this point of view too.
In the case of wooden golf club heads, relatively low specific gravity special to wooden materials allows appreciable enlargement in sweet spot without any serious increase in weight. Nevertheless, it is difficult with wooden materials to expect stable supply of constant quality and, sometimes, reliable supply of sufficient amount. Such unstable features in supply system is quite unsuited for mass-production. In addition, it is difficult to fix the position of the center of gravity in a club head stably. Further, due to the above-described limitation in weight, the volume of a wooden club head has an upper limit of, at highest, about 190 cc. and such a limited increase in size does not assure enlargement in sweet spot of a desirable extent.
CFRP club heads allow free increase in size, i.e. in volume. However, their relatively low moment of inertia near 2,700 g*cm.sup.2 allows no free enlargement in sweet spot as in the case of the wooden club heads.
In major cases of production, a club head has a face of 8 mm thickness (t1), a crown of 3 mm thickness (t2) and a sole of 10 mm thickness (t3) including a sole plate. It is in particular difficult to produce a club head having a face which is thin enough to allow large elastic flexion. Poor flexion of the face causes poor mating of the face with a ball which, as a consequence, cannot fly over a long distance in an intended direction.
From the foregoing, sufficiently enlarged sweet spot is obtained in a metallic club head of a hollow construction which has a face of 2 to 3.5 mm thickness (t1), a crown of 0.6 to 2.0 mm thickness (t2), more preferably of about 1.5 mm thickness and a sole of 1 to 3 mm thickness (t3). It should be appreciated that such a hollow construction of the club head enables sufficient enlargement in sweet spot without any corresponding serious increase in weight.
Conventional metallic club heads are in general made of fine cast material which is significantly large in specific gravity. It is infeasible with such cast material to produce a hollow construction with a thin shell because of various process demands in production. Flow of molten metal must be kept correctly in mould used for production and generation of cast defects must be prevented in order to present continuous mechanical strength in the product. These process demands all hinder formation of a thin shell for the light weight, large hollow construction of the club head. Further, increase in size, i.e. in volume results in undesirable increase in weight because of the relatively high specific gravity of the metallic materials.
When the thickness of the club head main body falls short of 3.5 mm, its face cannot endure impact at striking balls. In order to cover this deficit, it is necessary to insert into the face section a plurality of ribs in a matrix or honeycomb arrangement. In addition when such ribs of, for example, 1.5 to 2.0 mm thickness are inserted into the face section, cast defects and/or segregation are apt to be generated during casting process. Not only such a trouble in production, presence of such ribs in the face degrades flexion of the face at striking balls, which poses no sufficient repulsion on balls to be striken by the face. Such poor repulsion results in significant difference in behaviour time between elastic flexion of the face and elastic deformation of the ball. As a consequence, there is little coincidence between the recovery forces of the face and the ball resulted from their elastic recovery. Thus flexion of the face cannot be effectively utilized for deformation recovery of the ball and enhancement in speed of the ball after striking, thereby causing short fly of the ball in an unintended direction.
When a club head main body is made of stainless steel cast material, the material has a moment of inertia in a range from 2400 to 2500 g*.sup.2 and a specific inertia value "moment of inertia/specific gravity" of the material in a range from 300 to 320. These values are too small for the material to be used for a club head main body. As a conventional, the moment of inertia of a golf club head is a measure of the resistance to twisting or angular acceleration of the golf club head about its center of gravity.
In connection with the conventional club head materials, their proof stress (GPa) and density or specific gravity (g/cm.sup.3) are given in Table 1 whereas their size, weight, volume, moment of inertia and the specific inertia value "moment of inertia/specific gravity" are given in Tables 2 and 3. In Table 2, a height "a" refers to the distance between the crown and the sole, a width "b" refers to the distance between the toe and heel sides of the face and a length "c" refers to the distance from the face to the back of a club head main body.
TABLE 1 ______________________________________ Proof stress Density Material (GPa) (g/cm.sup.3) ______________________________________ Persimmon 20 0.8 Carbon 180 1.6 Stainless (steel cast) 90 7.9 Ti alloy (cast) 80 4.5 Ti alloy (rolled) 120 4.5 ______________________________________
TABLE 2 ______________________________________ Dimension in mm. Weight Volume Material t1 a b c in g. in cc. ______________________________________ Persimmon (rigid) -- 42 81 82 197 187 Carbon (hollow) 8 42 81 82 195 245 8 41 73 74 201 194 Stainless steel cast 3.1 36 68 71 205 148 (hollow) 3.1 41 69 71 204 168 3.0 40 71 71 190 170 Ti alloy cast 3.3 40 74 83 200 206 ______________________________________
TABLE 3 ______________________________________ Moment of Specific Use of Material inertia (g*cm.sup.3) inertia value ribs ______________________________________ Persimmon (rigid) 1900.about.2000 2500 none Carbon (hollow) 2625 1640 none 2458 1540 none Stainless steel cast 2471 310 2 mm (hollow) 2490 315 2 mm 2426 307 1.5 mm Ti alloy cast 3204 712 none ______________________________________